Optical transmission assembly may be divided into single-mode optical transmission assembly and multi-mode optical transmission assembly. From an overall product or device standpoint, optical signal transmission assembly may include Optical Subassembly (OSA) and Electrical Subassembly (ESA). Optical subassembly may further be divided into Transmitter Optical Subassembly (TOSA) and Receiver Optical Subassembly (ROSA).
A receiver optical subassembly (ROSA) that uses thin-film filter as key element often includes multiple optical signal channels, such as four (4) channels, with a spacing between the channels being typically around 0.5 mm to 0.75 mm. Due to difficulty and associated loss in making and assembling thin-film filters of smaller than 0.5 mm, by conventional structure and manufacturing approach, it is difficult to make optical filtering device assembly with smaller channel spacing. Optical filtering device assembly with smaller channel spacing may be made using waveguide divider that is based on planar lightwave circuit (PLC). However, planar lightwave circuit or waveguide has relatively low channel isolation and requires complicated manufacturing process. Because of high technical threshold of entry and manufacturing cost, PLC-based devices are generally suitable for dividers or filtering devices of high channel count, but not suitable for those devices of relatively low channel count.
In general, spacing in optical filtering device assembly determines the spacing of photo-diode array in a corresponding electrical subassembly (ESA). Usually, a bigger spacing of photo-diode array is coupled with great manufacturing difficulty, low yield, and high cost. Therefore, there is a need to manufacturing optical filtering device or assembly with smaller spacing to facilitate the use and manufacturing of electrical subassembly with smaller photo-diode array spacing thereby reducing the overall cost of corresponding optical subassembly (OSA).